Japanese Patent Application Publication No. 2010-40878 (FIGS. 1 and 2, Paragraphs 35 and 40) discloses an LED lighting device for controlling a current flowing through a light emitting diode (LED) by a step-down chopper circuit operating in a so-called boundary mode. Here, the boundary mode is a control mode in which, as shown in FIG. 10, an energy accumulated in an inductance element during an ON period TON of a switching element is discharged during an OFF period TOFF of the switching element, and the switching element is turned on again at a timing when the energy is completely discharged. The boundary mode has high power conversion efficiency compared to other control modes. Further, since half of a peak value of a switching current is an effective value of a load current, a constant current control can be easily realized.
For example, a case where a switching element Q1 of a step-down chopper circuit 1a as shown in FIG. 8A is operated in the boundary mode will now be described. Supplied between input terminals A and B is a direct current (DC) voltage obtained by boosting a voltage from, e.g., a commercial alternating current (AC) power source via a step-up chopper circuit. Connected between output terminals C and D is an LED series circuit or a load circuit including a plurality of LED series circuits connected in parallel. When the switching element Q1 is turned on, a current IQ1 shown in FIG. 10 flows through the switching element Q1→an inductor L1→a capacitor C2 to accumulate energy in the inductor L1. When the switching element Q1 is turned off, a counter electromotive force is generated by the energy accumulated in the inductor L1, and a flyback current ID1 flows through a path including the inductor L1→the capacitor C2→a diode D1. When the switching element Q1 is turned on again at a timing when the flyback current ID1 returns to zero, the power conversion efficiency is higher compared to other control modes since a switching loss is small and a pause period of the current does not occur.
In Paragraphs 35 and 40 of Japanese Patent Application Publication No. 2010-40878, there has been proposed a technique for PWM controlling the ON period TON of the switching element Q1 in the step-down chopper circuit 1a based on an external dimming signal. However, as can be seen from an operation waveform shown in FIG. 10, in case of narrowing the ON period TON of the switching element Q1, in conjunction therewith, an OFF period of the switching element Q1 is also narrowed. Consequently, an operating frequency of the switching element Q1 increases, and thus, the number of switching times increases, thereby resulting in an increase in the switching loss. Further, in case of expanding a range of the operating frequency, it makes it difficult to design a filter circuit for removing a switching noise.